Plate Heat Ex Changers

Plate Heat Exchangers A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperature change. Plate heat exchangers are now common and very small brazed versions are used in the hot-water sections of millions of combination boilers. The high heat transfer efficiency for such a small physical size has increased the domestic hot water (DHW) flowrate of combination boilers. The small plate heat exchanger has made a great impact in domestic heating and hot-water. Larger commercial versions use gaskets between the plates, smaller version tend to be brazed. The concept behind a heat exchanger is the use of pipes or other containment vessels to heat or cool one fluid by transferring heat between it and another fluid. In most cases, the exchanger consists of a coiled pipe containing one fluid that passes through a chamber containing another fluid. The walls of the pipe are usually made of metal, or another substance with a high thermal conductivity, to facilitate the interchange, whereas the outer casing of the larger chamber is made of a plastic or coated with thermal insulation, to discourage heat from escaping from the exchanger.

Design of plate and frame heat exchangers The plate heat exchanger (PHE) was invented by Dr Richard Seligman in 1923 and revolutionised methods of indirect heating and cooling of fluids. Dr Richard Seligman founded APV in 1910 as the Aluminium Plant & Vessel Company Limited, a specialist fabricating firm supplying welded vessels to the brewery and vegetable oil trades.Schematic conceptual diagram of a plate and frame heat exchanger. An individual plate for a heat exchanger The plate heat exchanger (PHE) is a specialized design well suited to transferring heat between medium- and low-pressure liquids. Welded, semi-welded and brazed heat exchangers are used for heat exchange between high-pressure fluids or where a more compact product is required. In place of a pipe passing through a chamber, there are instead two alternating chambers, usually thin in depth, separated at their largest surface by a corrugated metal plate. The plates used in a plate and frame heat exchanger are obtained by one piece pressing of metal plates. Stainless steel is a commonly used metal for the plates because of its ability to withstand high temperatures, its strength, and its corrosion resistance. The plates are often spaced by rubber sealing gaskets which are cemented into a section around the edge of the plates. The plates are pressed to form troughs at right angles to the direction of flow of the liquid

which runs through the channels in the heat exchanger. These troughs are arranged so that they interlink with the other plates which forms the channel with gaps of 1.31.5 mm between the plates. The plates produce an extremely large surface area, which allows for the fastest possible transfer. Making each chamber thin ensures that the majority of the volume of the liquid contacts the plate, again aiding exchange. The troughs also create and maintain a turbulent flow in the liquid to maximize heat transfer in the exchanger. A high degree of turbulence can be obtained at low flow rates and high heat transfer coefficient can then be achieved. A plate heat exchanger consists of a series of thin, corrugated plates which are mentioned above. These plates are gasketed, welded or brazed together depending on the application of the heat exchanger. The plates are compressed together in a rigid frame to form an arrangement of parallel flow channels with alternating hot and cold fluids. As compared to shell and tube heat exchangers, the temperature approach in a plate heat exchangers may be as low as 1 °C whereas shell and tube heat exchangers require an approach of 5°C or more. For the same amount of heat exchanged, the size of the plate heat exchanger is smaller, because of the large heat transfer area afforded by the plates (the large area through which heat can travel). Expansion The WCR stainless steel or titanium plate heat exchanger is available in sizes from 1/2" to 14" ports capable of handling flows up to 7,500gpm in one unit.

Evaluating plate heat exchangers All plate heat exchangers look similar on the outside. The difference lies on the inside, in the details of the plate design and the sealing technologies used. Hence, when evaluating a plate heat exchanger, it is very important not only to explore the details of the product being supplied, but also to analyze the level of research and development carried out by the manufacturer and the post-commissioning service and spare parts availability. An important aspect to take into account when evaluating a heat exchanger are the forms of corrugation within the heat exchanger. There are two types: intermating and chevron corrugations. In general, greater heat transfer enhancement is produced from chevrons for a given increase in pressure drop and are more commonly used that intermating corrugations.

Advantages

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Compactness- The units in a plate heat exchanger occupy less floor space and floor loading by having a large surface area that is formed from a small volume. This in turn produces a high overall heat transfer coefficient due to the heat transfer associated with the narrow passages and corrugated surfaces. Flexibility- Changes can be made to heat exchanger performance by utilizing a wide range of fluids and conditions that can be modified to adapt to the various design specifications. These specifications can be matched with different plate corrugations. Low Fabrication Costs- Welded plates are relatively more expensive than pressed plates. Plate heat exchangers are made from pressed plates, which allow greater resistance to corrosion and chemical reactions. Ease of Cleaning- The heat exchanger can be easily dismantled for inspection and cleaning (especially in food processing) and the plates are also easily replaceable as they can be removed and replaced individually. Temperature Control- The plate heat exchanger can operate with relatively small temperature differences. This is an advantage when high temperatures must be avoided. Local overheating and possibility of stagnant zones can also be reduced by the form of the flow passage.

Disadvantages •

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The main weakness of the plate and frame heat exchanger is the necessity for the long gaskets which holds the plates together. Although these gaskets are seen as drawback, plate-and-frame heat exchangers have been successfully run at high temperatures and pressures. There is a potential for leakage. The leaks that occur are sent to the atmosphere and not between process streams. The pressure drop that occurs through a plate heat exchanger is relatively high and the running costs and capital of the pumping system should be considered. When loss of containment or loss of pressure occurs, it can take a long time to clean and reinitialise this type of exchanger as hundreds of plates are common in larger builds.

Plate heat transfer equation •

The total rate of heat transfer between the hot and cold fluids passing through a plate heat exchanger may be expressed as: Q = UA∆Tm where U is the overall heat transfer coefficient, A is the total plate area, and ∆Tm is the temperature difference. U is dependent upon the heat transfer coefficients in the hot and cold streams.

Plate Heat Exchangers: Efficiency and Flexibility •

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Easy to Remove and Clean You simply remove the tie bolts and slide back the movable frame part. Now the plate pack can be inspected, pressure cleaned, or removed for refurbishment if required. Expandable A very significant feature of the plate heat exchanger is that it is expandable. Increasing your heat transfer requirements means simply adding plates instead of buying a new heat exchanger, saving time and money. High Efficiency Because of the pressed patterns in the plates and the relative narrow gaps, very high turbulence is achieved at relative low fluid velocity. This combined with counter directional flow results in very high heat transfer coefficients. Compact Size As a result of the high efficiency, less heat transfer area is required, resulting in a much smaller heat exchanger than would be needed for the same duty using other types of heat exchangers. Typically a plate heat exchanger requires between 20-40% of the space required by a tube & shell heat exchanger. Close Approach Temperature The same features that give the plate heat exchanger its high efficiency also makes it possible to reach close approach temperatures which is particularly important in heat recovery and regeneration applications. Approach temperatures of 1ºF are possible. Multiple Duties in a Single Unit The plate heat exchanger can be built in sections, separated with simple divider plates or more complicated divider frames with additional connections. This makes it possible to heat, regenerate, and cool a fluid in one heat exchanger or heat or cool multiple fluids with the same cooling or heating source. Avoid cross contamination Each medium is individually gasketed and as the space between the gaskets is vented to the atmosphere, cross contamination of fluids is eliminated. Less Fouling Very high turbulence is achieved as a result of the pattern of the plates, the many contact points, and the narrow gap between the plates. This combined with the

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smooth plate surface reduces fouling considerably compared to other types of heat exchangers. Lower Costs High heat transfer coefficients mean less heat transfer area and smaller heat exchangers, and sometimes even less heat exchangers. This and less space requirements, reduced flow rates, and smaller pumps means.